The diatoms, a unique class of microalgae, are enclosed in an extracellular, silica-based porous structure called the frustule. The frustule is denser than water and reduces cell buoyancy, thus increasing the risk of sinking out of the sunlight illuminated zone. Yet, diatoms are an outstanding ecological group with respect to global abundance, species diversification, and photosynthetic efficiency. The potential adaptive role of the diatom frustule can be linked to its light modulating properties, which have been explored as templates or raw materials for potential industrial, mainly photonic application. The optical properties of diatom frustules have also led to speculations about potential photo-biological relevance. Two main functions have been proposed: the frustule 1) modulates photosynthetic light harvesting by wave-diffraction or forward scattering via its nano-scale lattice structure and 2) reduces exposure to harmful radiation, i.e., by attenuation of ultraviolet radiation or mitigation of high light intensities. This chapter explores the unique photobiology of diatoms summarizing pigmentation and biochemical and structural adaptation of photosynthesis, including motility of chloroplasts and entire cells. We also discuss frustule photonics in relation to cellular light modulation for efficient light harvesting and highlight their potential implications for niche differentiation under various light regimes in the different habitats of diatoms.